It is not easy to print on fast-moving documents such as checks and deposit tickets, especially if they vary in thickness. One commonly used type of printing mechanism passes a document between a print head and a platen or hammer. Such a mechanism depends on a particular spacing or "gap" between the print head and platen or hammer. If the spacing is too small, unwanted debossing of the document (deformation of the document due to pressure from print wheels and the like) may occur. If the spacing is too great, there may be unprinted or faintly printed regions called "voids".
In the type of printer having a hammer which moves relative to a character formation element such as a print wheel, a controlled amount of energy is put into the hammer and this energy is absorbed by the paper and ribbon sandwiched between the hammer face and print wheel. Part of the energy is restored to the hammer by rebounding, as the system is partially elastic. The print quality is thus a function of hammer energy, character face area, and paper/ribbon characteristics. It is known to adjust the hammer energy based on character face area, and to adjust the energy to allow for the (unchanging) thickness of the paper presently loaded to the printer.
In present-day business activities, there is a premium set upon ever-faster printing, and upon ever-increasing numbers of documents processed between voids or other failures. As a result optimal gap adjustment is of increasing importance.
If the paper being printed upon is supplied as a continuous blank or preprinted form, it is often possible to set the gap once and leave it unchanged for the duration of the print run. If the paper being printed upon is in the form of individual sheets, and if the sheets have uniform thickness and other relevant physical characteristics, it is likewise often possible to set the gap once and leave it unchanged for the duration of the print run. However, where the documents to be printed upon vary in thickness from one to the next, a printer that has been set with a particular gap may encounter the above-mentioned embossing and voiding problems.
Several techniques for adjustment of printer gap are known. One known technique, typified in U.S. Pat. Nos. 4,575,267 to Brull and 4,632,577 to Brull et al., is simply to print on the document only after pressing the platen against the print head with a spring-loaded apparatus. Variations in the thickness of the document are taken up by varying distances of compression of the springs. While such an arrangement may accommodate varying document thickness in some printing applications, it has the drawback of requiring that the print head be moved repeatedly some distance away from the document and toward the document, once for each newly presented document.
Another technique, limited in its applicability to certain impact-type printers is taught in U.S. Pat. No. 4,173,927 to Van Kempen et al. The patent describes a printing apparatus having a rotating character drum and a print hammer. A print hammer is caused to accelerate toward the drum at such time as a desired character will be in place for printing. A detector is used to determine how long it takes for the hammer to reach the paper and drum. If this interval, called the "flying time", is deemed to be too long or too short, the drive parameters of the hammer, such as its initial position and driving force, are adjusted. One disadvantage of this apparatus is that when conditions change, at least one (typically poor) print must be made before the system can provide the necessary compensating adjustment.
Yet another known approach to the problem of accommodating changes in thickness of the print medium is exemplified by U.S. Pat. No. 4,088,215 to Bader and U.S. Pat. Nos. 4,174,908 and 4,233,895 to Wehler. In the Bader and Wehler apparatus, for example, the print head is adjustably linked to the platen, and a rider linked to the print head and located in its vicinity follows the print medium. The rider, having a pressure sensor, will yield a nearly constant output if the moving print medium remains constant in thickness. If the print medium becomes thicker or thinner, the output from the pressure sensor changes the changed output, which is constantly compared to a reference level, gives rise to an error signal. The error signal is amplified and drives a servo that adjusts the spacing or gap between print head and platen. The servo drives the gap size in the direction that reduces the error signal to a null level.
Wehler senses pressure on the rider by means of a megnetoresistor forming two legs of a Wheatstone bridge driving a differential amplifier. Bader uses a moving magnet in the proximity of a Hall-effect sensor. In either case, the sensor is quite nearby to the print head. Response to changes in record carrier thickness is quite quick limited only by the response time of the amplifier and motor, a few tens of milliseconds. The system drives to a null value at the amplifier input and output, and discards any information about the absolute thickness of the record carrier.
Still another approach is exemplified by U.S. Pat. No. 4,676,675 to Suzuki et al. Suzuki et al. teaches the use of an elastomeric material to form the active face of a pressure sensor. The sensor may be used to determine whether paper is present or not, and may also be used to determine the print gap size in connection with paper of a given thickness. The apparatus moves the print head and sensor toward the platen until the pressure has built up to a predetermined level, and then stops.
A related approach is seen in U.S. Pat. No. 4,652,153 to Kotsuzumi et al. and Pat. No. 4,812,059 to Masaki. The references each describe a method for setting a print head position in a dot-matrix printer. The print head is moved toward the paper, and thus toward the platen, until it physically contacts the paper and stops. The print head is then moved away from the paper to a predetermined distance. As a result, variations in thickness of the paper are accounted for. This method has the drawback that it requires substantial and discrete print head movements at the time of gap adjustment. The technique does not lend itself to use on a continuous basis for a long paper of varying thickness, nor is it well suited to handle discrete records of differing thickness at high record handling speeds.
The above-described approaches offer numerous drawbacks, and none is quite satisfactory for the high-speed presentation of discrete records. Where discrete records are to be presented at high speeds, one known approach is to employ multiple paper paths. For example, one high-speed paper path may be sorted into four paper paths for printing, each of which need only perform quickly enough to handle its portion of the stream. After the documents have been printed, the four paths are rejoined. Such an approach, though it permits use of slower printers, has many drawbacks. All the documents must be decelerated for the separate slower paper paths, and reaccelerated to rejoin the fast path. The acceleration and deceleration are fraught with jamming risks. Also, there are race conditions associated with the rejoining, aggravated by any variation in document length among the documents.
It would be desirable to have an apparatus capable of handling discrete records at high speeds. It would also be desirable if the apparatus could sense the need to vary the print gap in advance of the need for the variation, rather than sensing changes at the print head when it may be too late to correct for an interval within which there has already been some poor quality printing.